1. Field of the Invention:
The present invention relates generally to half-tone generation techniques and more particularly to the expansion of grey scale values in digitally controlled flying-spot scanning systems.
2. Description of the Prior Art:
Recent developments in optical imaging techniques have included flying-spot scanning systems in which a highly collimated light beam, preferably from a laser, is reflected by a driven reflecting surface to create a scanning light beam which transverse a focal plane imaging surface in a selected recurrent manner. Between scanning beams, the imaging surface itself is moved so that the complete imaging surface may be covered by successive scanning beams. The intensity of the scanning beam may be modulated in response to an analog or a digital signal to create an overall desired optical image.
The fly-spot scanning system has been applied to information reproduction systems such as xerography by the photo-receptive surface of a xerographic drum as the focal plane for the scanning beam. A limitation of the xerographic system, however, has heretofore precluded the reproduction of varying tone images because of the inability of the xerographic drum to accurately and consistently produce image tones other than black and white. Hence, in xerographic reproduction work, the intensity of the scanning light beam is, essentially, either "on" or "off".
Thus, there has long been a need in the field of xerographic reproduction for a means for producing variable tone images such as photographs or the like. The present invention satisfies that need.